1. Field of the Invention
The present invention relates to a method for modifying a chromosomal gene of Corynebacterium which is used for the production of useful substances such as amino acids by fermentation, resulting in a modification of its genetic character.
2. Discussion of the Background
Many attempts have been made to modify the genetic character of Corynebacterium by genetic engineering techniques and utilize the same for production of useful substances such as amino acids by fermentation. However, these methods all utilize a plasmid capable of autonomous replication as a vector (Japanese Patent Application Laid-Open Nos. 58-192900, 58-21699).
Plasmids replicate and function outside the chromosome. Therefore, where the genetic character of a bacterium is modified using plasmid DNA, the modification cannot be extended to the chromosome of the host bacterium. In particular, where DNA properties of the host bacterium, such as deletion, mutation, etc. come at issue, it has been impossible to make purposeful changes. To date, there is a method which comprises treating a host bacterium with a mutagen and then selecting a strain modified at random which has the desired properties, but this method involves much labor and many difficulties. Also, where plasmid DNA is used, the plasmid is often unstable and is expelled from the cell so that sufficient expression cannot be obtained or, many copies of the plasmid bearing the desired gene are present, whereby expression becomes excessive and such adversely affects the growth of bacteria or production of a substance. An object of the present invention is to modify a specific gene on the chromosome along the plan and fixing a definite number of copies of the gene stably on the chromosome, thereby to solve these problems.